1. Field of the Invention
This invention relates to a process for producing highly stereospecific .alpha.-olefin polymers. More particularly, this invention relates to a process for producing highly stereospecific .alpha.-olefin polymers quite low in the contents of catalyst residue and amorphous polymer and excellent in mechanical properties and processability by the use of a novel catalyst system having a very high catalyst activity per solid catalyst and per titanium atom.
2. Description of the Prior Art
As the method for producing a polymer of .alpha.-olefin such as propylene, butene-1 and the like, the method using the so-called Ziegler-Natta catalyst comprising a compound of transition metal belonging to Group IV-VI of the periodic table and an organometallic compound of a metal belonging to Group I-III of the periodic table is well known.
Production of .alpha.-olefin polymer is accompanied by formation of amorphous polymer in addition to the highly stereospecific .alpha.-olefin polymer having a high industrial value. This amorphous polymer is poor in industrial value, and it exercises greatly adverse influences upon the mechanical properties of processed articles, such as molded product, film, fiber and the like, of .alpha.-olefin polymer. In addition, the formation of amorphous polymer results in a loss of starting monomer, and it necessitates to provide a particular equipment for removing the amorphous polymer, which is quite disadvantageous from the industrial point of view. Accordingly, the catalyst system used in the production of .alpha.-olefin polymer must be a catalyst system which yields no amorphous polymer at all or yields only a vry slight quantity of amorphous polymer. Further, in the .alpha.-olefin polymer produced by such a process, residue of catalyst components comprising transition metal compound and organometallic compound remains, which makes various troubles on stability, processability, etc. of the resulting .alpha.-olefin polymer. Thus, a particularly equipment must be provided for removing the catalyst residue and stabilizing the polymer.
This disadvantage can be overcome by enhancing the catalyst activity expressed by the weight of .alpha.-olefin polymer formed per unit weight of catalyst. By taking this measures, the above-mentioned equipment for removing catalyst residue becomes unnecessary and production cost of .alpha.-olefin polymer can be reduced.
It is known that a supported catalyst prepared by supporting tetravalent titanium halide on magnesium halide can realize a certain extent of high-stereospecificity, high-activity polymerization of .alpha.-olefin when used in combination with an organoaluminum compound (co-catalyst) and organic silicon compound (the third component of catalyst) [Japanese Patent Application Kokai (Laid-Open) Nos. 57-63310 (EP-A-0045975), 58-83006 (GB-2111066), 61-78803]. It is also known that, in case of composite solid catalysts prepared by reducing a tetravalent titanium compound with organomagnesium to form an eutectic crystal, a high-stereospecificity, high-activity polymerization of .alpha.-olefin can be realized by the same combination as above [Japanese Patent Application Kokai (Laid-Open) No. 61-218606 (USP 4672050)].
Although both the above-mentioned two processes have reached a level capable of realizing no-extraction, no-deashing process to a certain extent, their further improvement is desired. Concretely speaking, realization of a more highly stereospecific polymerization is particularly desired for enhancing the quality of .alpha.-olefin polymer. Particularly in the field requiring a high stiffness of polymer such as the field of molded articles, a catalyst system having an ability to form a highly stereospecific polymer (for example, a catalyst system yielding 1.1% or less of CXS in the homopolymerization of propylene, wherein CXS means cold xylene-soluble fraction in total polymer yield) is earnestly desired, because realization of high stereospecificity directly leads to a quality of high stiffness.